In general, a dimmer is an apparatus designed to physically adjust brightness of an incandescent lamp and for a long time has served as a lighting device capable of controlling light due to its characteristics described below.
FIG. 1A and FIG. 1B show a basic operation of the dimmer, VAC refers to an alternating current (AC) power voltage, and VDIM refers to an output voltage of the dimmer. When the AC power voltage is phase-cut by the dimmer, the differential output voltage is divided into regions having the same value as that of a region having a positive or negative value as shown in FIG. 1A. At this time, in order for the dimmer to be turned on normally, a current over a specific value must flow in a region, a voltage value of both ends of which is a positive or negative value. When the voltage is a positive or negative value, and the current is zero or under the specific value, the dimmer is turned off in a corresponding cycle, the output voltage is discharged.
Furthermore, the incandescent lamp has an electrical resistance property because electric power consumed in a tungsten filament is converted into heat and light, and thus when a voltage is applied to both ends of the incandescent lamp, it enables current to flow.
Accordingly, the incandescent lamp very well satisfies a condition that “current should flow when a voltage value required to a dimmer operation is present at both ends.”
However, the incandescent lamp has high energy consumption, and accordingly, lamp technologies adopting a light emitting diode (LED) as a power source for energy saving have been continuously developed.
The light-emitting diode (LED) is a current drive device, and may normally operate when a constant current is stably supplied. In particular, since a drive current of the LED, which needs high electric power, is large (normally, 350 mA or more), a lot of heat is generated from the LED itself, and accordingly, a deterioration rate of luminance is larger than that of the LED at low electric power. This is directly associated with the LED's life span and acts as a very important factor in the lighting market.
For such a reason, the LED at high electric power is generally driven by a constant current, and here, a pulse width modulation (PWM) method is used so that the voltage of a switched-mode power supply (SMPS) used as the power supply of a first constant current can be more efficiently used.
However, due to the LED's characteristic of being driven by a constant current, additional circuits and various electronic components constituting the corresponding circuits are required for this method, and this leads to an increase in manufacturing costs of the lamp.
For such a reason, a method of rectifying an alternating current power source and applying it to a parallel-connected LED module is used, and LED lighting in which this method is used refers to AC direct type LED lighting. Accordingly, hereinafter, in the description regarding such an LED lighting type, the term “AC direct type LED lighting” will be used.
FIG. 2 is a view showing one example of conventional alternating current (AC) direct type LED lighting, and as illustrated in the drawing, a VAC voltage is outputted as a voltage VDIM in a phase-cut form by passing through the dimmer. Furthermore, the voltage VDIM passes through a rectifier, and thus a differential voltage thereof is converted into a single ended VRECT.
Furthermore, according to a level of the AC voltage converted into a common ground voltage, the VAC voltage is operated in a state of being divided into the operation sections of LED1 and CH1, LED1+LED2 and CH2, LED1+LED2+LED3 and CH3. At this time, even in the case of drive circuits in a form in which a separate LED current control unit is not provided, the same principle that the number of LEDs and corresponding channel current sources are combined according to a level of the input AC voltage and are driven is applied.
FIG. 3 shows operational waveforms at the time of applying a leading edge type dimmer to the AC direct type LED lighting according to FIG. 2, shows a case in which steepness of the operational waveforms of the dimmer is relative high, and shows voltage waveforms and current waveforms by steps and a normal or abnormal status of operation by sections.
As illustrated in the drawing, when the value of I_LED is more than a specific value, VDIM and VRECT waveforms follow waveforms of the Ac power source, whereas when the waveforms of VDIM and VRECT enter the section in which the value of I_LED is low, the waveforms of VDIM and VRECT cause malfunction regardless of the waveform of the AC power source. Due to this, LED currents are not maintained until the value of an AC voltage is reduced to zero, and the dimmer is turned off in advance. This is shown as a phenomenon in which a charge of a parasitic capacitor component being present in a dimmer output is naturally discharged to a leakage current path as a discharge path of the charge disappears. Accordingly, the value of VRECT is maintained at a high level, and thus minute electric currents also flow in a direction of the LEDs.
At this time, since a slope value of the dimmer is sufficiently large, brightness of the LEDs in a normal operation section is very high compared to that of the LEDs in an abnormal section, and accordingly, a user can feel that an operational status of the LEDs is a normal status without a large problem.
However, as shown in FIG. 4, when the wave steepness of a dimmer operation is relative low, an abnormal operational status of the LEDs is clearly recognized by the user.
That is, unlike FIG. 3, FIG. 4 shows voltage waveforms and current waveforms by stages and existence or non-existence of normal operations by sections when the wave steepness of the dimmer operation is relative low.
As illustrated, when all channels of the LEDs should be turned off because a dimmer wave steepness is low, the dimmer and drive circuits are abnormally operated in the entire region, and at this time, LED leakage currents generated due to a residual voltage of a parasitic capacitor component being present in the dimmer output cause an incorrect operation in which LED lighting is maintained in a state of being not turned off while shining dimly even in a dimmer angle section in which the LED lighting should be turned off.
In other words, the conventional AC direct type LED lighting causes a phenomenon generally called a flicker phenomenon which is one of very unsuitable factors in using the lighting lamp.